EP0145512B1 - Retractible blocking device to limit the flapping of the blades of a rotorcraft's main rotor - Google Patents

Description

The present invention relates to devices with retractable stops intended to limit the beating of the blades of a main rotorcraft rotor at low or zero speeds of rotation of the rotor, and the invention relates more particularly to a device of this type intended for the equipment of a rotorcraft rotor comprising a hub comprising an upper plate and a lower plate, and coupled to the foot of each rotor blade by means of at least one articulation device on which the thrust axes, drag and incidence of the corresponding blade intersect at a single point, and which comprises a single pin connecting to the plates and retaining the blade corresponding to the hub.

We know that when a helicopter rotor is stopped or rotates at low speed, for example when the rotor is rotated before takeoff, or when it slows down before it comes to a complete stop, after l landing, the blade is no longer subjected only to centrifugal forces and to aerodynamic lift forces which are zero or very reduced, so that the result of these forces on each of the blades is less than the weight of the blade, so that the latter pivots down around its beat joint. The weight of each blade therefore stresses the articulation device as well as the part of the hub on which this device is fixed, and, in particular when the articulation device consists of a laminated spherical bearing, the weight of the blade tends to deform this laminated spherical bearing around the flapping axis. This may result in the end of one or more of the blades of such a rotor striking the tail of the helicopter, its cabin or even possibly the ground, which may result in significant damage, in particular when the rotor rotates at low speed. On the other hand, rotors of this type are often fitted with relatively flexible blades, which, when the rotor stops or when it rotates at low speed, are very sensitive to wind gusts. However, these are particularly violent on certain marine platforms, floating or fixed, in particular for underwater oil research, on which such helicopters must land, as well as on ships provided with a landing surface. for helicopters. It is obvious that the forces of the wind on the blades stopped or turning slowly further increase the risk of collisions between the ends of the blades and parts of the helicopter or of the landing platform or landing surface from which it operates. .

For these reasons, a large number of devices with retractable stops intended to limit the beating of the blades of helicopter rotors have already been proposed.

In particular, the patent of the United States of America No. 2,614,640 corresponding to the characteristics of the characterizing part of claim 1, describes independent lower stops for the different blades of a main helicopter rotor.

Each independent lower stop is carried by a first lever, mounted pivoting about a substantially horizontal axis, fixed to the hub. On the lower part of this first lever are articulated two second levers, carrying lower weights, and respectively coupled to the first lever by means of a spring. The arrangement of these elements is such that, when the rotor stops, the spring maintains each second lever in an almost vertical position, in which it immobilizes the first lever, carrying the stop, in a higher position, where said stop can limit the deflection of the corresponding blade downwards, while, when the rotor turns at a sufficient speed, the centrifugal force applied to each counterweight, opposing the action of the spring, causes the second levers to disappear, which therefore, allow the pivoting down of the first lever, and the retraction of the corresponding stop.

U.S. Patent No. 2,719,593 also describes independent lower stops for the various blades of the main rotor of a helicopter. Each independent stop essentially comprises a bent lever, the first arm of which forms a stop, and the second arm carries a counterweight. The angled lever is mounted oscillating on a pivot fixed horizontally to the rotor hub. A spring acts on the pivoting lever so as to keep it, when the rotor stops, in a position where its first arm limits the lowering of a part secured to the driven articulation of the corresponding blade, so as to limit the subsidence of the latter. The two arms of the bent lever form an angle such that the centrifugal force, which is applied to the counterweight when the rotor turns, tends to overcome the force of the spring and to pivot said lever in a direction such that the stop of its first arm moves away downwards from said piece integral with the corresponding blade, so as to retract the stop.

U.S. Patent No. 2,465,674 describes independent lower stops for the various blades of a main helicopter rotor. Each lower stop is carried by a first lever mounted pivoting about a substantially horizontal axis, fixed to a plate forming the hub of the rotor. A counterweight is pivotally mounted about another horizontal axis, on a plate, itself sliding below said plate, towards which it is pushed by a spring. A second lever, carrying this counterweight, is itself coupled to the first lever, carrying the stop, so as to pivot said first lever, for a sufficient speed of the rotor, to a position for erasing the corresponding stop. When lowering the swashplate, used for adjustment of the pitch of the different rotor blades, a vertical pusher, actuated by the swash plate, pushes the sliding plate downwards by compressing the corresponding spring, and this plate rotates the first lever, so as to bring the corresponding stop in a service position that also makes it take the second lever carrying the counterweight, when the speed of the rotor decreases. This stop system is therefore very complicated and unreliable.

U.S. Patent No. 4,368,006 also describes independent lower stops for the different blades of a rotor, each stop comprising two pendulum arms each mounted around a horizontal axis on a part projecting radially towards the outside of the drag axis support of the corresponding blade. Each arm carries, at one of its ends, a counterweight and has, at its other end, a rounded cam, centered on the pivot axis, and which engages, when the pendulum arm is brought back into position by a spring, at low or zero speeds of the rotor, against a rounded support surface of complementary shape to a pad integral with the connection structure at the foot of the blade, and which also has an inclined ramp extending the support surface and intended to prevent any jamming of the shoe against the cam of the stopper, in the intermediate position, while in the retracted position by centrifugal force, a flat surface of the shoe can come to bear against a flat stopper surface of the projecting part of the support of the drag axis. Each pendulum arm also has two stops by each of which it comes into contact with one of two other abutment surfaces of the projecting part of the support of the drag axis, in the two extreme positions of the pendulum arm.

These various known embodiments all have the drawback of not lending themselves to a favorable adaptation on the hubs essentially consisting of two superposed plates perpendicular to the axis of rotation of the rotor, because they imply the presence of numerous added elements, having numerous abutment surfaces, such as pivot axes, levers, weights, springs, etc. arranged in complex arrangements, bulky and relatively unreliable, protruding below the lower plate, which is unfavorable in terms of aerodynamic drag, weight and cost.

Finally, the lower stops independent of the known embodiments and presented above are inoperative to oppose the movements of the blades, when, when the rotor stops or at low speeds of rotation thereof, the blades are biased towards the high by a strong gust of wind. In this case, the blade thus stressed can take a high angle of conicity and then fall heavily on the lower stopper, possibly causing a deterioration of this lower stopper and by itself undergoing an instantaneous bending moment much higher than the loads it can normally cash out, which can lead to its decommissioning.

In order to remedy this latter drawback, it has already been proposed, in particular by French Patent No. 1,368,684, to equip a rotorcraft rotor with independent upper and lower stops for the different blades, the production of each of these stops , lower and upper, being close to those described in the first two patents of the United States of America cited above. However, the adaptation of such an embodiment to a hub essentially consisting of plates leads to doubling the complexity, the weight and the cost which characterize the embodiments comprising only independent lower stops.

Furthermore, it has already been proposed, in French patent n ° 2434079, an equally complex embodiment at the level of the independent upper stops, since each of these comprises an angled lever whose first arm forms a stop and whose second arm carries a counterweight, this bent lever being mounted oscillating on a pivot fixed on the upper part of the rotor hub, parallel to the axis of the latter, and in the vicinity of the axial plane of symmetry of the corresponding blade at rest, and this lever being biased by a spring which keeps it, when the rotor is stopped, in a position such that its first arm extends substantially parallel to the axial plane of symmetry of the blade, the two arms of the bent lever forming an angle such that the centrifugal force, which is applied to the counterweight when the rotor turns, tends to overcome the force of the spring and to pivot the lever in a direction such that the stop of its first arm moves away from the foot of the pay. This embodiment is all the more complex since a member fixed on the upper part of the hub of the rotor is provided to absorb the shocks and the forces received by the stop of each pivoting lever, this member being for example a crown fixed on the part upper of the hub, concentrically with the axis of the rotor, and arranged to support the pivots of all the levers. However, such an embodiment, complex at the level of the upper stops, is acceptable insofar as it is combined with a simple embodiment of the lower stops, which consist of a common lower stop for all the blades and of the reciprocal ring type. We know that a reciprocal ring constitutes a much simpler, cheaper and more reliable lower stop than independent lower stops for the different blades, but it has the disadvantage that, if a gust of wind occurs during the critical phases of starting or rotor shutdown, as often happens, especially when a helicopter must land on or take off from a marine platform or a ship, in an environment where the winds are strong and swirling, the gust of wind strongly pushes down one of the blades, which then presses violently on the reciprocal ring, so that the latter, not being subjected to reaction forces sufficient on the part of the other blades, takes an extreme position in which it does not prevent said blade, violently pushed down, from reaching a dangerous position, of strong inclination, and possibly coming to strike or even cut by its end the tail or cabin of the helicopter.

The present invention aims to provide a device with retractable stops, independent, limiting the flapping movement of the blades down and preferably also up, which is well suited to the equipment of the hubs with two plates, in particular of the type in which each blade is connected to the hub by an articulation device for which the three axes of articulation of each blade intersect at a single point, and which comprises a single pin connecting to the plates and retaining the blade corresponding.

One of the aims of the invention is to propose a device of the aforementioned type which is mechanically more compact, simpler and more robust than the known embodiments, while being lighter and less expensive than the latter, and having a drag reduced aerodynamics.

Another object of the invention is to propose a device with retractable stops which is particularly advantageous when the articulation device of each blade to the hub is a laminated spherical bearing, in order to allow the operation of the pitch of the blades, at low or zero speeds of the rotor, as well as to facilitate the folding of the blades after the rotor has stopped, without damaging the laminated spherical bearings, and without requiring, for the folding of the blades, special tools such as that described for example in French patent n ° 2,423,396.

Yet another object of the invention is to provide a device with retractable stops offering an improved capacity for stopping and launching the rotor in high winds, in particular for helicopters having to operate from ships or platforms at sea. , without having the drawback of reciprocal rings and without however being as disadvantageous as systems with stops independent of the achievements of the state of the art.

To this end, the stop device according to the invention, intended to limit the beating of the blades of a main rotorcraft rotor of the type presented above, that is to say comprising an upper plate and a lower plate, and coupled to the foot of each blade by means of at least one articulation device on which the axes of beat, drag and incidence of the corresponding blade intersect at a single point, and which comprises a single pin connecting to the plates and retaining the blade corresponding to the hub, is a device with stops comprising, for each blade. an independent lower stop, consisting of a support member and a counterweight carried at the end of an arm of the support member, and pivotally mounted on the hub, as well as a lower support secured to the blade root or a connecting member connecting the blade root to the articulation device, and located under said blade root or said connecting member, the lower stop being subjected to the action of a return spring of so that the position of the lower stop results from the balance between the centrifugal force and the elastic return force of the spring, depending on the rotational speed of the rotor, and that at low or zero rotor speeds, the spring recalls the lower stop in a position of cooperation of the support member and of the lower seat, in order to limit the beating of the blade downward, while at high speeds of the rotor, the centrifugal force retracts the lower stop at l against the spring by rotating it, so that the organ support is not opposed to the downward beatings of the blade, and is characterized in that the support member has substantially the shape of a curved roller which has a circular recess by which it is pivotally mounted around from the axis of the single blade retaining pin corresponding to the hub, above the lower plate, and a part projecting outwards relative to the axis of the circular recess and at least partially delimited by a convex abutment surface against which the lower surface is intended to come into contact.

Advantageously, in order to be able to check whether the flight controls are in good condition, without damaging the lower stops, the device according to the invention is further characterized in that, on each lower stop, the convex stop surface extends, in a direction perpendicular to the axis of the retaining pin, over the entire pitch range of the corresponding blade, between two extreme positions which correspond, one to the minimum pitch obtained when the cyclic and collective steps are minimum, and l '' other, at maximum pitch, obtained when the cyclic and collective pitch are maximum, and so that for an average pitch, obtained when the cyclic pitch is zero and the collective pitch small, and when the rotor stops, the contact between the lower bearing surface and the convex abutment surface takes place in an intermediate position, so that at low or zero speeds of the rotor, the contact between the lower bearing surface and the convex abutment surface is not broken when the inclination of the pale es t modified by the operation of the flight controls. In this case, in fact, the bearing can cause the curved roller when the pitch of the blade is changed, and the roller can therefore move on either side of its equilibrium position to cover the pitch range.

In order to constitute an extreme downward limit stop for each blade at high rotor speeds, the projecting part of the corresponding support member is extended, on one side of the surface of the convex stop, by a substantially rectilinear against which the corresponding lower bearing can come to bear when the corresponding blade tends to take a too sharp negative angle risking bringing it into contact with the fuselage of the rotorcraft.

Advantageously, so that the convex abutment surface of each support member can adapt to the different angular deflections of the corresponding blade without being damaged, the abutment surface is such that its intersection with a plane perpendicular to the axis of the corresponding pin or with a plane passing through the axis of this pin has substantially the shape of an arc of a circle, the center is located on the side of the spindle axis.

In a simple embodiment, each lower bearing has a planar track of contact with the corresponding convex abutment surface, this planar track being perpendicular to the axis of incidence of the corresponding blade. However, it is also possible that each lower bearing surface has a concave track for contact with the corresponding convex abutment surface, the concave track then having a radius of curvature greater than that of the corresponding convex abutment surface.

Each lower stop is dimensioned to be retracted by centrifugal force preferably, when the speed of the rotor reaches substantially half of the nominal rotation speed.

In a preferred embodiment, which is intended for equipping a rotor whose articulation device of each blade to the hub is a laminated spherical bearing comprising a rigid internal radial element, movable relative to the plates, and on which a rigid loop is retained, continuously bypassing the laminated spherical bearing and integral with the resistant framework of the corresponding blade or a connecting member secured to the foot of the corresponding blade, as well as an external rigid radial element, fixed between the edges of the plates, in the manner of a spacer, and consisting of a sleeve, the bore of which is crossed by the single connecting pin, around which the corresponding blade can be pivoted, the device with retractable stops according to l the invention is further characterized in that the support member of each lower stop is pivotally mounted by its circular recess around an anti-friction ring mounted with a tight fit around the end lower tee of the corresponding sleeve and on which is hooked one end of the return spring of the lower stop.

This embodiment, which has many advantages specified below, is more particularly intended for use on hubs with plates connected to the rotor blades by spherical laminated bearings directly integrated into the base of the blades, as described in the application for French patent n ° 81 22 027 (FR-A-25 16 891 But it is also possible to use such an embodiment on hubs, the two plates of which are connected by each laminated spherical bearing to a radial yoke connecting to the foot of the blade corresponding, said yoke preferably being double and hollowed out in its part facing the hub to allow free passage of the bearing, as described in the request for French addition certificate n ° 7911 585 (FR-A-24 56 In these different hubs, the position of each drag blade is determined, in particular at low or zero rotor speeds, by an elastic drag return strut with incorporated damping. preferably having the form of an organ consisting of an alternating stack of rigid metallic or composite plates and plates of a visco-elastic material with great resistance to deformation and having its internal and external ends each coupled by a ball joint respectively at the hub and at the foot of the corresponding blade, the center of the ball joint of the external end being close to the axis of beating of the blade, which passes through the center of the corresponding laminated spherical bearing.

With regard to the production of the hubs, it is possible that the central barrel of the hub and the upper plate constitute a first metal monobloc element fixed, for example, by a ring of bolts to the lower plate itself constituting a second metal monobloc element with the upper part of the rotor mast, but it is also possible that the central barrel of the hub, integral with the upper plate, is fixed, still by a crown of bolts, to the lower plate, itself integral with the upper part of the rotor mast . Finally, it is possible that, in accordance with French patent application No. 8211988 (FR-A-25 29 860), the hub comprises two plates made of composite materials and comprising a laminated flange, constituted by a stack of layers of fabrics of fibers with high mechanical resistance, surrounded by a belt of rovings of fibers with high mechanical resistance.

An important advantage of the embodiment according to the invention and presented above is that, when folding a blade, it is the assembly consisting of the blade, the laminated spherical bearing and the lower stop which pivots around the axis of the single pin connecting to the plates and retaining the blade. We know that the spherical laminated bearings are made to work essentially in compression, and that, during the folding of a blade, a significant component of the lateral force is introduced into the alternating metallic and elastomer layers of the corresponding laminated bearing, and stresses these layers in shear, therefore in poor conditions. This drawback is eliminated in the device according to the invention, due to the overall pivoting mentioned above around the blade retaining spindle, the latter remaining in support and moving, via the track its lower bearing, on the convex stop surface of the lower stop.

In order to reduce the risk of damage to the rotor when, when stationary or turning at low speeds, it is subjected to strong gusts of wind, which can tend to raise the blades around their beating axis, the device with stops retractable according to the invention comprises. in addition, for each blade, an independent upper stop, limiting the movement of said blade upward consisting of a support member and a counterweight carried at the end of an arm of the support member, and pivotally mounted on the hub, as well as an upper bearing secured to the blade root or to a connecting member connecting the blade root to the articulation device, and located on said leg blade or said connecting member, the upper stop being subjected to the action of a return spring so that its position results from the balance between the centrifugal force and the elastic return force of the spring, depending on the speed of rotation of the rotor, and that at low or zero speeds of the rotor, the spring recalls the upper stop in a position of cooperation of the support member and the upper bearing, in order to limit the beating of the blade upwards , while at high speeds of the rotor, the centrifugal force retracts the upper stop against the spring by rotating it, so that the support member does not oppose the upward beating of the blade.

By analogy with the production of the lower stops, the support member of each upper stop also preferably has the shape substantially of a curved roller which has a circular recess by which it is pivotally mounted about the axis of the single blade retaining pin to the hub, below the upper plate, and a part projecting outwards relative to the axis of the circular recess and at least partially delimited by a cylindrical abutment surface centered on the axis of the spindle, and against which the upper bearing is intended to come into contact.

Such an embodiment of separate high and low stops articulated on the blade retaining axis is of a simple and light design, and has the advantage that the high and low stops are all housed between the two plates, this which is favorable for good compactness and for transmission without deformation of the significant support forces. The operating principle of the high stops is identical to that of the low stops, except that it is not necessary to ensure the continuity of contact of the high stops with the corresponding upper spans, during blade pitch movements, at speeds weak or no rotor. The role of the upper stops is to limit the range of movement of the blades, on the ground, when starting or stopping the rotor.

In order to increase the efficiency of the high stops, each upper bearing preferably has a flat track of contact with the corresponding cylindrical stop surface, said track being perpendicular to the axis of incidence of the corresponding blade. In addition, each upper stop is retracted by centrifugal force when the speed reaches substantially a quarter of the nominal rotation speed.

By analogy with the lower stops, the support member of each upper stop is preferably pivotally mounted, by its circular recess, around an anti-friction ring mounted with tight fit around the upper end of the sleeve and on which is hooked one end of the return spring of the corresponding upper stop.

However, according to a second embodiment, to which preference should be given, the device according to the invention further comprises, for each blade, a second lower counterweight pivotally mounted relative to the hub about an axis perpendicular to the axis of the corresponding spindle and to the axis of incidence of the blade, and having a spoiler-shaped part turned towards the spindle and which is engaged, at low or zero rotor speeds, in a notch presented by the corresponding lower bearing, on the side opposite to the spindle, under the effect of the elastic return exerted radially and inwards by a second spring on the second counterweight moved towards the lower bearing, in order to block the beating of the blade towards the top, while at high rotor speeds, the second counterweight is retracted radially outward by centrifugal force, which rotates it against the second spring and releases it from the lower seat, so that it cannot oppose the upward beating of the blade. This variant therefore has a combined set of upper and lower stops mounted around the blade retaining axis and grouped at the base of the connection of the blade to the hub. In this variant, each second counterweight, which is integrated into the system constituted by the lower stop and the corresponding lower span, is spread, in flight, by centrifugal force and allows the blade to take off from its support on the convex roller of the corresponding lower stop, via the corresponding lower bearing. When the rotor is stopped, this second counterweight therefore serves as an upper stop, because of its spoiler-like structure, at its upper part, which engages in the corresponding notch formed in the face of the lower bearing which is facing the blade. In this embodiment, the blade cannot escape upwards under the action of the wind, even when the latter is violent and blows in gusts. It is the same when starting or stopping the rotor, respectively before or after the blade is subjected to a centrifugal force which outweighs its weight.

In addition, in order to allow the pitch movements of the blades, the notch of each lower bearing advantageously has a concave profile of revolution around the axis of incidence of the corresponding blade. Preferably, when the lower span is supported by its track against the convex abutment surface, the spoiler is received with play in the corresponding notch, this small play allowing an easier engagement and retraction of the spoiler under the action of centrifugal force.

In a preferred embodiment, the second counterweight is carried at the outer radial end of a yoke, the two arms of which are pivoted on either side of a heel projecting radially outwards which has a surrounding ring. , under the lower plate, the lower end of the corresponding pin, the second spring being a coil spring, longitudinal and tension, which extends between the two arms of the yoke and connects the second counterweight to the heel. In this case, it is preferable that the lower end of the spindle carries an enlarged retaining head, having a flat cooperating with a shoulder of the heel, in order to prevent any rotation of the spindle.

Whether the upper stops are integrated or not with the lower stops, in order to give the device maximum efficiency under the effect of centrifugal force, on the one hand, and to allow, on the other hand, the free tilting of the bearing laminated spherically during the flapping of the corresponding blade, the position of the counterweight of each lower stop and / or, if necessary, of each upper stop, relative to the axis of the corresponding spindle, is inclined by about 45 ° on the axis of incidence of the corresponding blade and inwards with respect to the axis of the spindle.

Likewise, whatever the design adopted for producing the upper stops, each lower and / or possibly upper stop is elastically returned by a tension spring with a longitudinal coil, extending substantially radially with respect to the axis of the spindle, between the corresponding counterweight and a fixed anchoring point relative to the hub, or by a spiral tension spring, which surrounds the spindle and is hooked by one end to the corresponding support member and by the other end at a fixed anchor point relative to the hub.

Finally, if each blade of the rotor comprises a stirrup-shaped pitch control lever, the branches of which extend transversely, one above and the other below the blade root or the foot connecting member of corresponding blade, the corresponding lower surface of the device according to the invention is preferably fixed to the lower face of the lower branch of the stirrup, and, where appropriate, the corresponding upper surface is fixed to the upper face of the branch top of this stirrup.

• la figure 1 représente une vue en coupe partielle verticale passant par le plan médian d'une pale. d'un rotor équipé d'un premier exemple de dispositif à butées escamotables selon l'invention,
• les figures 2 et 3 sont des vues en coupe transversale partielle du rotor représenté sur la figure 1, respectivement selon les lignes II-II et 111-III,
• les figures 4 et 5 sont des vues analogues à la figure 1 d'un moyeu de rotor équipé d'un deuxième exemple de dispositif à butées escamotables selon l'invention, respectivement dans les positions occupées à l'arrêt et en vol.
• la figure 6 est une vue en coupe selon VI-VI de la figure 4,
• la figure 7 est une vue en perspective avec coupe et arrachement partiel représentant une butée haute et basse combinée du second exemple de réalisation,
• les figures 8 à 11 représentent schématiquement en coupe transversale quatre positions différentes d'une butée basse du premier exemple de réalisation conforme à l'invention,
• les figures 12 et 13 représentent respectivement, en coupe transversale et de manière schématique, les positions au sol et en vol d'une butée supérieure du premier exemple conforme à l'invention, rappelée par un ressort à spirale,
• les figures 14 et 15 sont des figures analogues aux figures 12 et 13 pour une butée supérieure rappelée par un ressort à boudin longitudinal, et
• les figures 16 et 17 sont des vues schématiques en coupe transversale représentant respectivement les positions au sol et en vol d'une butée inférieure du second exemple de réalisation conforme à l'invention.

The present invention will be better understood with the aid of the description given below, without implied limitation, of exemplary embodiments with reference to the appended drawings in which:

• Figure 1 shows a vertical partial sectional view passing through the median plane of a blade. a rotor equipped with a first example of a device with retractable stops according to the invention,
• FIGS. 2 and 3 are views in partial cross-section of the rotor shown in FIG. 1, respectively along lines II-II and 111-III,
• Figures 4 and 5 are views similar to Figure 1 of a rotor hub equipped with a second example of device with retractable stops according to the invention, respectively in the positions occupied when stopped and in flight.
• FIG. 6 is a sectional view along VI-VI of FIG. 4,
• FIG. 7 is a perspective view with section and partial cutaway showing a combined upper and lower stop of the second embodiment example,
• FIGS. 8 to 11 show diagrammatically in cross section four different positions of a bottom stop of the first embodiment according to the invention,
• FIGS. 12 and 13 respectively represent, in cross section and schematically, the positions on the ground and in flight of an upper stop of the first example according to the invention, recalled by a spiral spring,
• FIGS. 14 and 15 are figures similar to FIGS. 12 and 13 for an upper stop recalled by a longitudinal coil spring, and
• Figures 16 and 17 are schematic cross-sectional views respectively representing the positions on the ground and in flight of a lower stop of the second embodiment according to the invention.

The structure of the helicopter rotor hub partially shown in Figures 1 to 3, as well as the structure of the blades of this rotor and their connection to the hub are as described respectively in French patent applications 82 11 988 and 81 22 027. It will simply be recalled that the hub shown in FIG. 1 essentially comprises three elements, which are a spacer (not shown). generally cylindrical, in line with the rotor mast, and two identical plates, one (2) of which is the upper plate and the other (3) of the lower plate. The two plates (2) and (3) are fixed to the spacer in the manner described in French patent application 82 11 988, which will advantageously be referred to for more precision. While the spacer and the members for fixing the latter to the two plates (2 and 3) are metallic elements, the two plates (2 and 3) are made of composite materials. Each plate (2 and 3) comprises a laminated flange (4) constituted by a stack of layers of fabrics of fibers with high mechanical resistance and surrounded by a belt (5) of rovings of fibers with high mechanical resistance. Each flange (4) has a star shape having as many branches as the rotor blades. In addition, each branch of the star flange (4) is wider on the side of the central portion of the flange (4) than on the side of its external radial end (6), with flat upper and lower faces, which is advantageously enlarged and rounded and has a bore in which is mounted a shouldered ring (7). The belt of rovings (5) in carbon fibers for example, and which surrounds each flange (4), is pressed against the edge of the branches of this flange (4) while being received in cells open towards the outside in the edge external radial ends (6) of each branch of the flange (4), then the rovings belt (5) is externally protected by an additional sheet of fabric (8). As shown in FIG. 1, the identical upper (2) and lower (3) plates, extending in planes substantially perpendicular to the axis of rotation of the rotor, are arranged so that the respective branches of the two plates (2 and 3) are exactly superimposed and that their shoulder rings (7) are coaxial.

A laminated spherical bearing, of a known type, playing simultaneously the role of drag articulation, beat and incidence (for controlling the pitch), and the assembly of which is designated by (9) is mounted between the ends (6) of each pair of superimposed branches of the two plates (2 and 3). The functions of the bearing (9) are well known: it takes up the centrifugal forces and the radial forces exerted on the blade to which it is connected, with small deformations, and it allows the angular movements of steps, with an elastic return limited, as well as the oscillations of beat and drag of the blade. These characteristics result from the sandwich structure of the central part (10) of this bearing (9), consisting of an assembly of metal cups, in spherical portions, alternated with thin layers of elastomer which has good fatigue strength. and of excellent quality of adhesion to metals, two reinforcements, one of which (11) has a concave spherical surface and the other (12) a convex spherical surface, being adhered by molding to the central part (10) of the laminated spherical bearing (9). The frame (12), in the external radial position, and having the convex spherical surface, consists of a metal sleeve whose external wall has two bulges (12a and 12b) in the form of a hollow spherical cap, centered on the longitudinal axis of the blade. The sleeve (12) is placed between the ends of the superimposed branches of the upper (2) and lower (3) plates, provided with their shouldered ring (7), and connects them to each other by means of a single pin (13). This hollow metal pin (13) passes through the rings (7) and a set of two anti-friction shoulder rings (14) and a bush (15) forming a spacer between the shoulder of the upper ring (7) and a washer. (16) anti-friction resting on the end of the corresponding branch of the lower plate (3). This pin (13) has an enlarged flat head (17) retained against an anti-friction washer (18) resting against the end of the corresponding branch of the upper plate (2), by tightening a nut (19) screwed and preferably braked on the threaded lower end of the spindle (13) projecting under the lower plate (3). At the ends of the central bore of the sleeve (12), are provided anti-friction shouldered rings (20) mounted with a tight fit therein and dimensioned so that their internal diameter has a slight play with respect to the external diameter rings (14) and that the stacking dimension of the sleeve (12) and of the two shoulders of the rings (20) also has a slight play with respect to the distance between the internal faces of the shoulder of the upper ring (7 ) and the lower washer (16).

This arrangement thus allows easy mounting and dismounting of the blade (21) on the hub (1) by a single pin (13) and, if necessary, allows without disassembly of said pin (13), the angular pivoting of the sleeve (12) around the axis of the spindle (13) to allow the "folding of the corresponding blade (21).

The other frame (11), in the internal radial position, of the laminated spherical bearing (9) is constituted by a metal block substantially in the form of a pyramid trunk in the large base of which the portion of concave spherical surface turned towards the spherical bulge (12a) of the sleeve (12), and in the small base of which has been formed a groove opening onto the lateral faces of the block 11 and turned towards the central spacer of the hub (1).

Each rotor blade (21) comprises rovings, each of which preferably consists of a set of basic threads, filaments or fibers of high resistance coated and agglomerated in parallel in a bundle by a hardened synthetic resin, and which constitute in the area at the foot of the blade (21) a continuous rigid loop (22) extending into the current part of the blade to gradually form the resistant spar, at the leading edge, as well as elements of the trailing edge. The two opposite parts of the roving loop (22) are separated from each other by a block 23 of a filling material, which delimits with the end part of the roving loop (22) a free space constituting a housing for the laminated spherical bearing (9) that the loop of rovings (22) continuously surrounds, coming to be housed in the groove of the internal frame (11), with interposition of layers protective fabrics (24) of high strength fibers, for example carbon fibers.

Between the bulge (12b) of the sleeve (12) and the wall of the filling block (23) which is turned towards the axis of the rotor, and which is preferably, at this level, reinforced by some rovings (22a) of the frame of the blade (21), is arranged a part (25) in two parts, having a concave spherical bearing surface (25a) located opposite the bulge (12b). The part (25) is fixed by means of two bolts (26), on the lower branch (27), extending transversely under the blade (21), of a pitch control lever of this blade, in stirrup shape whose upper branch (28) also extends transversely but above the blade (21). The wedge-shaped part (25) constitutes a holding and blocking device for the loop of rovings (22) at the bottom of the groove in the internal radial frame (11) of the bearing (9), mainly during launching. and / or stopping the rotor, with or without wind gust. The contact being established between the bulge (12b) of the sleeve (12) and the corner (25) avoids the untimely exit of the loop (22) from its housing and any harmful effect which could ensue. In addition, this arrangement facilitates mounting of the blade (21) and frees the bearing (9) from part of the vertical shearing force due to the weight of the blade (21) when the latter is no longer under the effect of centrifugal force. Bypassing the internal frame (11) of the bearing (9) by the roving loop (22) of the blade (21), the latter, the foot of which receives the bearing (9), can carry out all the necessary deflections around the center of this bearing (9) and the blade (21) can also pivot, for its folding, around the single pin (13) which retains it to the hub (1) by means of the bearing (9). Finally, the pitch control lever of each blade (21), which is essentially constituted in the form of a stirrup, the lower (27) and upper (28) branches of which extend transversely, respectively below and below above the two opposite parts of the rovings loop (22) and the filling material block (23) of the foot of the blade (21), is connected on the one hand, to the end of a pitch control rod and, on the other hand, at the outer end of an elastic return stretcher with incorporated damping, which is constituted in a well known manner, of an alternating stack of rigid plates and of plates of a visco-elastic material with great resistance to deformation, such as a silicone elastomer. The counter-plug (not shown) which makes it possible to drive the blades in rotation and to elastically counter their oscillations in drag in the plane of the rotor, and which also acts as a damper for these oscillations, as well as the control rod for steps (also not shown) are articulated on the pitch control lever by ball joints.

The rotor which has just been described is practically identical to those which are the subject of the last two above-mentioned French patent applications. However, it differs from these hubs in that it includes a device for retractable stops limiting the beating of the blades.

As shown in Figures 1 and 3, a bottom stop consisting of a support member in the form of a curved roller, (30) extending from an arm (31) carrying at its end a weight (32), is mounted pivoting around the lower end of the sleeve (12), above the lower plate (3), and pressing on the washer (16). The support member (30) has a recess of circular section in a thick central part (33) which is integral with an eccentric part (34) projecting outwardly relative to the axis of the recess circular. The eccentric part (34) is formed by a thin central veil surrounded by a rim which, in its most projecting part, is delimited by a convex external surface (35). As shown in Figure (3), it extends towards the counterweight (32) by an outer surface (36) substantially flat or very slightly curved. The convex surface (35) is such that its intersection with a plane perpendicular to the axis of the recess or of the spindle (13) or with a plane passing through this axis is substantially an arc of circle centered on the side of this axis . Through its recess, the support member (30) is rotatably mounted around a collar anti-friction bushing (37) which is fitted tightly around the lower cylindrical end of the sleeve (12). This socket (37) rests by its lower end against the shoulder of the ring (20), and its collar carries a boss (38) for hooking one end of a tension spring, with longitudinal coil, (39 ) the other end of which is hung on a central stud (40) carried on the substantially cylindrical counterweight (32). The assembly of the bottom stop is thus produced and mounted that at rest (Figure 3), the spring (39) extends radially relative to the axis of the spindle (13), along an inclined axis of approximately 45 ° on the axis of incidence of the corresponding blade (21), and inwards with respect to the axis of the spindle (13). The convex surface (35) constitutes an abutment surface against which the track (41) can bear, flat and perpendicular to the axis of incidence of the corresponding blade (21) which is present in the direction of the spindle (13 ), a lower seat (42), of square section, fixed against the lower face of the lower branch (27) of the stirrup pitch control lever by four bolts (43) which simultaneously secure the branches (27 and 28) of the lever to the blade (21) and the fixing, on the upper face of the upper branch (28), of an upper bearing (44). The latter presents, towards the pin (13). a track (45), also flat and perpendicular to the axis of incidence of the blade (21), which is intended to come into abutment against a cylindrical abutment surface whose axis is that of the spindle (13), and that has the thick and most projecting end (48) of an eccentric extension (47) of a support member (46) of a high stop which also includes an arm (49) integral with the member (46) and carrying a flyweight (50). The support member (46) of the upper stop has a recess of circular section, by which it is pivotally mounted around a sleeve or anti-friction collar ring (51) which, by symmetry with the ring or sleeve (37) of the lower stop, is adjusted tightly around the upper cylindrical end of the sleeve (12), and is pressed by its upper end against the shoulder of the ring (20), so that the high stop is pivotally mounted around the axis of the spindle (13), below the upper plate (2). The ring (51) has a boss (52) for hooking one end of a tension spring (53) with a longitudinal coil, the other end of which is hooked on a central stud (54) under the counterweight (50) . As for the lower stop, at rest (Figure 2) the spring (53) extends radially with respect to the axis of the spindle (13), along an axis inclined by about 45 ° on the axis of incidence of the blade (21), and inwards relative to the axis of the spindle (13).

The operation of the device with stops which has just been described will now be explained with the aid of FIGS. 9 to 11 for the low stops and of FIGS. 14 and 15 for the high stops.

It is understood that the low and high stops, pivotally mounted, between the two plates (2 and 3), by their respective support member (30 and 46) around the pin (13) for retaining the blade (21), are of the retractable type under the effect of centrifugal force when the latter urges their respective counterweight (32 and 50), against respective return springs (39 and 53), which determine, at low or zero rotor speeds, the positions of the stops as shown respectively in Figures 3 and 8 for the low stops and in Figures 2 and 14 for the high stops .

When the rotor stops, the blade (21) is supported by the track (41) of its lower range, at the level of the pitch lever, against the convex abutment surface (35) of the bottom abutment which does not allow that a low angle of beat (about 1 °) towards the bottom of the blade (21) under the effect of its own weight and possibly a gust of wind, while, simultaneously, the high stop is in position for oppose any upward movement of a blade, due to a gust of wind which would immediately cause the track (45) of the upper span (44) to press against the cylindrical abutment surface (48) of the 'support member (46). When the rotor is stopped, the incidence of each blade normally corresponds to an average step because the cyclic step is zero and the collective step is in the small step position. The convex (35) and cylindrical (48) abutment surfaces of the low and high stops respectively are produced so that, in this configuration of the blade (21), the support against the tracks (41 and 45) of the lower spans (42 ) and upper (44) is in the intermediate position, between the ends of the abutment surfaces 35 and 48 in a direction perpendicular to the axis of the spindle (13). Indeed, as the three centers of articulation of the blade (21) are located in the center of the laminated spherical bearing (9), the bottom stop is concerned not only by the beat movements, but also by the movements of step and trail. As on the ground, the drag movements are negligible, because the position of the blade (21) is given by the elastic return stretcher, the movements of steps therefore condition the production of the convex abutment surface (35) . However, on the ground and rotor stopped, it is necessary to be able to maneuver the flight controls and to vary the pitch of the blades in order to carry out an adjustment of the flight controls and to ensure their good condition. In order to allow this adjustment and this verification, the convex abutment surface (35) of the support member in the form of a curved roller (30) of the bottom abutment extends, in a direction perpendicular to the axis of the spindle (13), over the entire pitch range of the corresponding blade (21). This convex abutment surface (35) is therefore presented as a cam which extends between two ends (55 and 56) (see FIGS. 8 to 11), one of which (55), on the side opposite the counterweight (32 ), is in contact with the track (41) of the blade (21) (FIG. 9), when the pitch of the latter is minimum, which corresponds to a minimum cyclic pitch (that is to say of negative value maximum) and a small collective pitch, while the other end (56), on the side of the counterweight (32), is in contact with the track (41) (FIG. 10) when the pitch of the blade (21) is maximum which corresponds to a maximum positive cyclic step and a large collective step.

The convex abutment surface (35) therefore ensures continuity of contact between the bottom abutment and the runway (41 on the ground, when the rotor is stopped, when the flight controls are operated to vary the pitch between these maximum and minimum values, as well as when the rotor is launched, when the latter begins to rotate. During these phases, the support member (30), driven by the track (41), can pivot around the spindle (13), on either side of its intermediate and equilibrium position in which the spring (39) recalls it, covering the pitch range, while remaining in support and moving on the track (41) .

The cylindrical abutment surface (48) of the upper abutment does not extend, in a direction perpendicular to the spindle (13), to as great an extent as the convex abutment surface (35), because it is not necessary to ensure continuity of the contacts between the upper stop and the track (45) of the upper bearing (44), during the movements of steps, because the essential role of the high stops is to limit upward the amplitude of the movement blades, on the ground, when the rotor stops and starts.

When the rotor is launched, the blades are stressed by centrifugal force and tend to tension radially by lifting from their support against the low stops. When the speed of the rotor reaches a quarter of the nominal speed, the centrifugal force which requests the weights (50) of the high stops prevails over the return of the springs (53), so that the high stops are retracted by rotation around the spindles (13). Each upper stop therefore passes from the position in FIG. 14 to that of FIG. 15. When the speed of the rotor reaches half the nominal speed. the centrifugal force which urges the weights (32) of the lower stops prevails over the return of the springs (39), so that the lower stops are in turn retracted by rotation around the pins (13) and each lower stop takes the position of figure 11. The position of the flyweights (32 and 50) at 45 ° relative to the axis of incidence of the blades gives the stops maximum efficiency under the effect of centrifugal force and allows free tilting of all of the bearings (9) during the beating of the blades. In flight, under the centrifugal effect and around the nominal speed of the rotor, the low stops authorize a downward beat of the blades of the order of 7 to 8 °. The position of the stops, depending on the rotor speed, is then given by the balance of the centrifugal forces, acting in particular on the counterweights and the elastic return of the springs.

When the rotor stops, it is first of all the low stops which are recalled in the position of limitation of the downward beating of the rotor, by the springs (39), then the high stops are recalled in the position of limitation of the beating towards the top, by the springs (53).

The coil tension springs (39 and 53) can be replaced by spiral springs, such as the spring (57), shown in Figures 12 and 13 on a high stop. This spiral spring (57) surrounds the upper end of the sleeve (12) and is anchored by one end curved in a notch in the support member (46), while its other curved end is anchored in a part integral with the sleeve (12), for example a ring such as (51) in FIGS. 1 and 2.

The abutment device described above is not only compact due to the arrangement of the low and high stops between the two plates of the hub, but it offers an important maintenance advantage: when folding a blade of the rotor towards the rear for example, the assembly constituted by the blade (21 the spherical bearing (9) and the corresponding lower stop is pivoted around the spindle (13). The folding is done simply by blocking the blade at incidence, at the using a simple tool placed between the spindle (13) which is hollow and a fixing point on the upper branch (28) of the corresponding pitch control lever, then by unhooking the elastic return strut against and then turning the blade by a suitable angle around the spindle (13).

The rotor shown in Figures 4 to 7 has many features in common with that which has been described with reference to Figures 1 to 3, so that only the differences between these two embodiments are worth mentioning. The rotor of FIGS. 4 to 7 is no longer equipped with a device with separate low and high stops, but with a device in which a combined assembly is grouped at the base of the connection of each blade to the hub and plays at the times the function of a high stop and a low stop. There is therefore no longer an upper support member (46) carrying an upper counterweight (50) and having a cylindrical abutment surface (48), and which pivots around the upper end of the sleeve (12) and cooperates with the track (45) of the upper bearing surface (44) of the blade (21). On the other hand, there is a lower stop identical to that previously described, pivoting by the circular recess of its support member (30) (in the form of a curved roller having the convex stop surface (35)) around the ring ( 37) clamped around the lower end of the sleeve (12), and whose weight (32) at the end of the arm (31) is returned by the spring (39). The convex abutment surface (35) is intended to cooperate with the lower surface (42) of the blade (21) in the same manner as in the previous example. But, in this example, the lower bearing (42) has, on the side opposite to its track facing the spindle (13). a notch (58) having a concave profile of revolution around the axis of incidence of the blade (21). This notch (58) is intended to cooperate with the spoiler-shaped end (60) of a second lower counterweight (59), shaped as a yoke pivotally mounted by its two arms (61) on either side of a heel (62). The latter, of rounded shape, projects radially outward and downward relative to the lower plate (3). The flyweight (59), which pivots about an axis (63), is retracted (FIG. 5), that is to say moved radially outward and away from the lower bearing surface (42), in flight, by the centrifugal force, and it is recalled radially inwards (FIG. 4), at low or zero speeds of the rotor, by a tension spring (64) which passes between the arms (61) of the yoke and is hooked to a clip (65) under the counterweight (59) and to a clip (66) on the heel (62). On the ground, the counterweight (59) immobilizes the movement of the blade upwards by the engagement of its spoiler (60) in the notch (58) of the lower bearing (42). The profile of revolution of the notch (58) allows the pitch movements of the blade (21) when the helicopter is on the ground. When the counterweight (59) is retracted, in flight, the blade (21) can be detached from its support by the lower bearing (42) against the convex abutment surface (35) of the bottom abutment, and as shown in Figures 5 and 17, the lower stop can then be retracted in turn by rotation of the support member (30) around the spindle (13). Conversely, when the speed of the rotor is reduced, the lower stop is first returned to position by the spring (39), then the blade (21) gradually pivots downward, until it comes to rest by its reach (42) against the lower stop, and finally the spring (64) recalls the counterweight (59) and engages the spoiler (60) in the notch (58), with a small clearance which is shown in FIGS. 4 and 16. On the ground, rotor stopped or when starting or stopping the rotor, the flyweight (59) therefore serves as a high stop which prevents the blade (21) from escaping under the action of a strong and continuous wind and / or gusts. The small clearance existing in this configuration between the spoiler (60) and the notch (58) allows the frank engagement and retraction of said spoiler in said notch. In this example, the spindle (13) has a lower end with an enlarged head (67) which is screwed into the main part of the spindle (13), and the enlarged head (67) has a flat by which it is locked in rotation in a countersink machined in a washer (68) integral with the heel (62) and pressing against the underside of the lower plate (3). This second embodiment, also very compact and of simple design, has the advantage of providing an appreciable gain in mass compared with the first solution described above, as well as compared with the embodiments known from the prior art.

In addition, when the blades are folded, the presence of the low and high stops combined below the spherical laminated bearings (9), in conjunction with the presence of the anti-recoil parts (25) of the blades (21), makes it possible to avoid that the laminated stops (9) are not stressed in tension which would be very unfavorable since these elements are designed to be stressed in compression. For the rest, the folding of a blade is carried out as easily as in the previous example, by an overall rotation around the corresponding pin (13).

Claims (23)

1. Device with retractable stops designed to limit the flapping of the blades (21) of a rotorp- lane's main rotor at low or zero speeds of rotation of the rotor, which device is of the type comprising a hub (1) which comprises an upper plate (2) and a lower plate (3) and is coupled to the root of each blade (21) by way of at least one articulation means (9) at which the flapping and drag axes and the axis of incidence of the corresponding blade (21) intersect at one point only, and which comprises a single pin (13) for connection to the plates (2, 3) and for retaining the corresponding blade (21) on the hub (1), the stop device comprising for each blade (21) an independent lower stop that is formed by a support member (30) and a counterweight (32) supported at the end of an arm (31) of the support member (30) and is pivotally mounted on the hub (1), together with a lower abutment (42) that is integrally formed with the root of the blade (21) or with a connecting element connecting the root of the blade (21) to the articulation means (9) and that is situated under the said root of the blade (21) or the said connecting element, the lower stop (30, 32) being subjected to the action of a return spring (39) so that the position of the lower stop results from the equilibrium between the centrifugal force and the resilient return force of the spring (39) as a function of the rotational speed of the rotor, and so that at low or zero speeds of the rotor the spring (39) returns the lower stop (30, 32) to a position of cooperation of the support member (30) and the lower abutment (42), so as to limit the downward flapping of the blade (21), whilst at elevated speeds of the rotor, the centrifugal force retracts the lower stop (30, 32) against the spring (39) by making it pivot so that the support member (30) does not oppose downward flapping of the blade (21), characterised in that the support member (30) is approximately in the form of a rounded disc that has a circular aperture by means of which it is pivotally mounted about the axis of the single pin (13) for retaining the corresponding blade (21) on the hub (1), above the lower plate (3), and that has a portion that projects outwardly from the axis of the circular aperture and is at least partially defined by a convex stop face (35) against which the lower abutment (42) is designed to come to bear.

2. Device with retractable stops according to claim 1, characterised in that on each lower stop (30, 32), the convex stop face (35) extends in a direction perpendicular to the axis of the pin (13), over the entire range of pitch of the corresponding blade (21), between two end positions (55, 56) of which one corresponds to the minimum pitch obtained when the cyclic and common pitches are minimum, and of which the other corresponds to the maximum pitch obtained when the cyclic and common pitches are maximum, with the result that for a medium pitch, obtained when the cyclic pitch is zero and the common pitch is small, and when the rotor stops, the contact between the lower abutment (42) and the convex stop face (35) occurs in an intermediate position, in order that at low or zero speeds of the rotor the contact between the lower abutment (42) and the convex stop face (35) is not broken when the angular position of the blade (21) about its longitudinal axis is modified by the operation of the flight controls.

3. Device with retractable stops according to claim 2, characterised in that at one side of the convex stop face (35) the projecting portion of the corresponding support member (30) is lengthened by an approximately rectilinear portion (36) against which the corresponding lower abutment (42) bears when the corresponding blade (21) tends to adopt too large a negative flapping angle at elevated speeds of rotation of the rotor.

4. Device with retractable stops according to one of claims 1 to 3, characterised in that the convex stop face (35) is such that its intersection with a plane perpendicular to the axis of the corresponding pin (13) or with a plane passing through the axis of this pin (13) is approximately in the shape of an arc of a circle of which the centre is situated on the side of the axis of the pin (13).

5. Device with retractable stops according to one of claims 1 to 4, characterised in that each lower abutment (42) has a planar contact face (41) for the corresponding convex stop face (35), the said face (41) being perpendicular to the axis of incidence of the corresponding blade (21).

6. Device with retractable stops according to one of claims 1 to 4, characterised in that each lower abutment (42) has a concave contact face for the corresponding convex stop face (35), the said concave face having a radius of curvature greater than that of the corresponding convex stop face (35).

7. Device with retractable stops according to one of claims 2 to 6, characterised in that the end (56) of the convex stop face (35) which, in the end position corresponding to the maximum pitch, is in contact with the corresponding lower abutment (42), is situated at the same side of the axis of incidence of the corresponding blade (21) as the counterweight (32), at low or zero speeds of the rotor.

8. Device with retractable stops according to one of claims 1 to 7, characterised in that each lower stop (30, 32) is retracted by the centrigual force when the speed of the rotor reaches approximately half of the nominal rotational speed.

9. Device with retractable stops according to one of claims 1 to 8, designed for a rotor of which the articulation means for each blade (21) on the hub (1) is a spherical laminated bearing (9) comprising an internal rigid radial element (11), movable in relation to the plates (2, 3), and on which there is held a fixed loop (22) passing continuously around the spherical laminated bearing (9) and integrally formed with the strong frame of the corresponding blade (21) or with a connecting element integral with the root of the corresponding blade, and also comprising an external rigid radial element (12) that is fixed between the edges of the plates (2, 3), in the manner of a spacer, and formed by a sleeve of which the bore is passed through by the single connecting pin (13) about which the corresponding blade (21) may be pivoted, characterised in that the support member (30) of the corresponding lower stop is pivotally mounted by way of its circular aperture about an anti-friction collar (37) mounted to fit closely about the lower end of the sleeve (12) and to which there is coupled one end of the return spring (39) of the lower stop (30, 32).

10. Device with retractable stops according to one of claims 1 to 9, characterised in that it further comprises, for each blade (21), an independent upper stop that is formed by a support member (46) and a counterweight (50) supported at the end of an arm (49) of the support member (46) and is pivotally mounted on the hub (1), together with an upper abutment (44) that is integrally formed with the root of the blade (21) or with a connecting element connecting the root of the blade to the articulation means (9), and that is situated on the said root of the blade (21) or the said connecting element, the upper stop (46, 50) being subjected to the action of a return spring (53) so that the position of the upper stop (46, 50) results from the equilibrium between the centrifugal force and the resilient return force of the spring (53), as a function of the rotational speed of the rotor, and so that at low or zero speeds of the rotor the spring (53) returns the upper stop (46, 50) to a position of cooperation of the support member (46) and the upper abutment (44) so as to limit the upward flapping of the blade (21), whilst at elevated speeds of the rotor the centrifugal force retracts the upper stop (46, 51) against the spring (53) by making it pivot so that the support member (46) does not oppose the upward flapping of the blade (21).

11. Device with retractable stops according to claim 10, characterised in that the support member (46) of each upper stop is also approximately in the shape of a rounded disc having a circular aperture by means of which it is pivotally mounted about the axis of the single pin (13) for retaining the blade (21) on the hub (1), below the upper plate (2), and a portion (48) that projects outwardly from the axis of the circular aperture and that is at least partially defined by a cylindrical stop face centred on the axis of the pin (13) and against which the upper abutment (44) is designed to bear.

12. Device with retractable stops according to claim 11, characterised in that each upper abutment (44) has a planar contact face (45) for the corresponding cylindrical stop face (48), the said face (45) being perpendicular to the axis of incidence of the corresponding blade (21).

13. Device with retractable stops according to one of claims 10 to 12, characterised in that each upper stop (46, 50) is retracted by the centrifugal force when the speed of the rotor reaches approximately a quarter of the nominal rotational speed.

14. Device with retractable stops according to one of claims 10 to 13, as appended to claim 9, characterised in that the support member (46) of the upper stop is pivotally mounted by way of its circular aperture about an anti-friction collar (51) mounted to fit closely about the upper end of the sleeve (12) and to which there is coupled one end of the return spring (53) of the upper stop (46, 50).

15. Device with retractable stops according to one of claims 1 to 9, characterised in that it further comprises, for each blade (21 a second lower counterweight (59) pivotally mounted in relation to the hub (1) about an axis perpendicular to the axis of the corresponding pin (13) and to the axis of incidence of the blade (21 and having a portion in the form of a beak (60) that faces the pin (13) and that is engaged, at low or zero speeds of the rotor, in a slot (58) in the corresponding lower abutment (42), on the side opposite to the pin (13), under the resilient return action, exerted radially and towards the inside, of a second spring (64) on the second counterweight (59) displaced towards the lower abutment (42), so as to block the upward flapping of the blade (21), whilst at elevated speeds of the rotor, the second counterweight (59) is radially retracted towards the outside by the centrifugal force which causes it to pivot against the second spring (64) and disengages it from the lower abutment (42) so as not to oppose upward flapping of the blade (21).

16. Device with retractable stops according to claim 15, characterised in that the slot (58) in the lower abutment (42) has a concave profile of revolution about the axis of incidence of the corresponding blade (21) so as to permit adjustment of incidence of the blade (21).

17. Device with retractable stops according to one of claims 15 and 16, characterised in that when the lower abutment (42) is bearing with its face against the convex stop face (35), the beak (60) is received in the slot (58) with some clearance.

18. Device with retractable stops according to one of claims 15 to 17, characterised in that the second counterweight (59) is supported at the outer radial extremity of a catch of which the two arms (61) are pivoted on both sides of a lug (62), projecting radially outwards, on a collar (68) that surrounds, under the lower plate (3), the lower end of the corresponding pin (13), the second spring (64) being a longitudinal coil tension spring which extends between the two arms (61) of the catch and connects the second counterweight (59) to the lug (62).

19. Device with retractable stops according to claim 18, characterised in that the lower end of the pin (13) carries an enlarged retaining head (67) having a surface plane cooperating with a shoulder of the lug (62) so as to prevent any rotation of the pin (13).

20. Device with retractable stops according to one of claims 1 to 19, characterised in that at low or zero speeds of the rotor, the position of the counterweight (32, 50) of each lower stop and/or, where applicable, of each upper stop, in relation to the axis of the corresponding pin (13) is inclined at approximately 45° to the axis of incidence of the corresponding blade (21) and towards the inside in relation to the axis of the pin (13).

21. Device with retractable stops according to one of claims 1 to 20, characterised in that each lower stop (30, 32) and/or, where applicable, each upper stop (46, 50) is resiliently returned by a longitudinal coil tension spring (39. 53) extending approximately radially in relation to the axis of the pin (13) between the corresponding counterweight (32, 50) and an anchorage point (38, 52) fixed in relation to the hub (1).

22. Device with retractable stops according to one of claims 1 to 20, characterised in that each lower stop (30, 32) and/or, where applicable, upper stop (46, 50) is resiliently returned by a spiral traction spring (57) which surrounds the pin (13) and is coupled, at one end, to the corresponding support member (30, 46) and, at the other end, to an anchorage point fixed in relation to the hub (1).

23. Device with retractable stops according to one of claims 1 to 22, characterised in that each blade (21) comprises a pitch control lever in the form of a stirrup of which one of the arms (28) extends transversely above and the other (27) transversely below the root of blade (21) or the connecting member to the corresponding blade, the corresponding lower abutment (42) being fixed to the lower face of the lower arm (27) and, where applicable, the corresponding upper abutment (44) being fixed to the upper face of the upper arm (28) of the lever.

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